1. Field of the Invention
This invention relates to a light controlling apparatus for enhancing the contrast ratio and maintaining the light quantity uniform.
2. Description of Related Art
Usually a polarizing plate is used for a light controlling apparatus which uses a liquid crystal cell. For example, a TN (Twisted Nematic) liquid crystal cell or a guest-host liquid cell (GH (Guest Host) cell) is used as the liquid crystal cell.
FIG. 7A and FIG. 7B are schematic diagrams for describing the operational principle of a conventional light controlling apparatus. The light controlling apparatus comprises mainly a polarizing plate 1 and a GH cell 2. The GH cell 2 is sealed between two glass plates. Liquid crystal molecules 3 and dichromatic dye molecules 4 are sealed in the GH cell 2. Dichromatic dye molecules 4 exhibits the anisotropic light absorption. In detail, the type of this dichromatic dye molecules 4 is p-type, which absorbs the light of major axis direction of a molecule. The liquid crystal molecules 3 exhibit the positive dielectric anisotropy.
FIG. 7A shows the GH cell 2 to which no voltage is applied (no voltage application). The incident light 5 is converted to a linear polarized light by passing through the polarizing plate 1. In FIG. 7A, because the polarization direction is identical with the major axis direction of dichromatic dye molecules 4, the light is absorbed by the dichromatic dye molecules 4 and the transmittance of the GH cell is reduced.
A voltage is applied as shown in FIG. 7B. Then, the major axis direction of the dichromatic dye molecules 4 is turned to the direction perpendicular to the defection of the linear polarized light. Therefore, the light is not absorbed by the GH cell 2.
If n-type dichromatic dye molecules, which absorb the light of the minor axis direction of the molecules, is used, the light is not absorbed when no voltage is applied and is absorbed when a voltage is applied contrary to the case of the above-mentioned p-type dichromatic dye molecules 4.
In the case of the light controlling apparatus shown in FIG. 7, the ratio of absorption under application of a voltage to the ratio under application of no voltage, namely ratio of optical density, is about 10. This optical density ratio is about twice that of a light controlling apparatus comprising only a GH cell without a polarizing plate 1.
In a conventional light controlling apparatus, a polarizing plate is placed on the optical path of the light fixedly. Therefore, 50% of light is absorbed by the polarizing plate consistently. Further, the surface reflection from the polarizing plate affects the light. The maximum light transmittance of the light which passes the polarizing plate does not exceeds 50%, and the reduction of light quantity is significant. The light quantity reduction is one of the factors which hinder the commercialization of the light controlling apparatus having a liquid crystal cell.
On the other hand, various light controlling apparatus which do not use a polarizing plate have been proposed. As an example of the light controlling apparatus having no polarizing plate, a light controlling apparatus having a two-layered GH cell has been known. The first layer of the GH cell absorbs the polarized component in the same direction as that of a certain polarized light and the second layer absorbs the polarized component in the perpendicular direction to that of the polarized light. As another example of the light controlling apparatus having no polarizing plate, a light controlling apparatus which utilizes the cholesteric-nematic liquid crystal phase transition has been known. Further, a polymer scattering type light controlling apparatus which utilizes liquid crystal scattering has been known.
However, these light controlling apparatus having no polarizing plate can not exhibit the ratio of absorbance under no voltage application to the ratio under application of a voltage, namely the ratio of optical density, which exceeds 5 as described herein above. As a result, since the contrast ratio of the light controlling apparatus is small, such light controlling apparatus are insufficient for normal light control in the environment ranging from a dark place to a bright place. The polymer scattering type light controlling apparatus is significantly poorer in the image-forming performance of the image sensing optical system.
The present invention was accomplished to solve the above-mentioned problems. It is the object of the present invention to provide a light controlling apparatus which enhances the contrast ratio and maintains the light quantity uniform.
To solve the above-mentioned problems, a light controlling apparatus in accordance with the present invention is provided with a liquid crystal cell and a polarizing plate which is movable inside/outside the effective optical path of the light incident to the liquid crystal cell.
Because the polarizing plate is movable outside the effective optical path of the light, the light is not absorbed by the polarizing plate, and the maximum transmittance of the light controlling apparatus is increased.
The polarizing plate is desirably made movable inside/outside the effective optical path by providing the polarizing plate on the movable portion of a mechanical iris.
Because the polarizing plate is moved out or in by means of the mechanical iris, the light controlling apparatus is realized easily.
The liquid crystal cell is desirably a guest-host type liquid crystal cell.
Because a guest-host type liquid crystal cell is used, the light is controlled by absorbance of the light by the guest-host type liquid crystal cell itself.
The polarizing plate is desirably made movable inside/outside the effective optical path by providing the polarizing plate on the movable portion of a mechanical iris, and the liquid crystal cell is desirably a guest-host type liquid crystal cell.
The liquid crystal cell desirably comprises a first guest-host type liquid crystal cell which is capable of switching between absorption and transmission of the same polarized component in the direction as the polarized component which is absorbed by the polarizing plate, and a second guest-host type liquid crystal cell which is capable of switching between absorption and transmission of the polarized component perpendicular to the polarized component which is absorbed by the polarizing plate.
Therefore, the first guest-host type liquid crystal cell absorbs the same polarized component as the polarized component which is absorbed by the polarizing plate. In the middle of moving in/out of the polarizing plate, the transmitted light quantity on the portion of the aperture where the polarizing plate does not cover is equalized to that of the portion of the aperture where the polarizing plate covers.
The liquid crystal cell desirably comprises first and second guest-host type liquid crystal cells which are capable of switching between absorption and transmission of the polarized component perpendicular to the polarized component which is absorbed by the polarizing plate.
Therefore, these two first and second guest-host type liquid crystal cells both absorb the light which passes the polarizing plate. As the result, the minimum transmittance of the light controlling apparatus is lowered.
The liquid crystal cell desirably comprises a first guest-host type liquid crystal cell which is capable of switching between absorption and transmission of the same polarized component in the direction as the polarized component which is absorbed by the polarizing plate, and second and third guest-host type liquid crystal cells which are capable of switching between absorption and transmission of the polarized component perpendicular to the polarized component which is absorbed by the polarizing plate.
Therefore, the first guest-host type liquid cell absorbs the same polarized component as the polarized component which is absorbed by the polarizing plate. Further, the second and third guest-host type liquid crystal cells both absorb the light which passes the polarizing plate. As the result, the first guest-host type liquid crystal cell functions to equalize the transmitted light quantify on the portion of the aperture where the polarizing plate does not cover to the transmitted light quantity on the portion of the aperture where the polarizing plate covers. Simultaneously, the second and third guest-host type liquid cells function to reduce the minimum transmittance of the light controlling apparatus more.